The present invention relates to charged particle accelerator devices, more particularly to resonant cavity structures used in particle accelerator devices for sustaining high acceleration fields.
Particle accelerator devices such as high-energy accelerators and free electron lasers (FEL) are designed to accelerate charged atomic particles such as ions, protons, or electrons. Acceleration of a beam of charged particles is traditionally accomplished using a synchronized linear radio frequency (RF) cavity array. Each cavity in such a linear array sustains a high-intensity RF radiation field in order to generate a high acceleration field. A synchronized acceleration electric field EACC is applied by each cavity along the linear charged-particle beam trajectory. The injected charged-particle beam acceleration is provided by the linear array whereby each cavity imparts an energy increment to the beam. When synchronized properly, these increments add up to the desired high velocity at the end of the linear array.
To maximize the accelerating electric field (EACC) in each such RF cavity, the cavity must sustain a high-power electromagnetic field and a minimum of resistive losses at the cavity's walls. The purpose of using superconductor material in the cavities of conventional particle accelerator devices is to minimize wall losses, thereby promoting long duty cycles. State-of-the-art RF cavities are made of a low temperature superconductor such as niobium (Nb). RF cavity arrays made of bulk Nb are commonly used for free electron lasers and large high-energy-physics-research accelerators. Niobium is the only conventional superconductor that meets the requirements of small surface resistance, high thermal conduction, and ductility; see Ruggero Vaglio, “RF Superconducting Cavities for Accelerators,” pp 447-473, Microwave Superconductivity, H. Weinstock and M. Nisenoff, Editors, Kluwer Academic Publishers, Dordrecht, 2001. However, by definition a low temperature superconductor material acts as a superconductor only at a low operation temperature, the achievement of which requires liquid helium (He) cooling. Use of niobium, for instance, mandates an operation temperature T in the range T=2-4 K, more commonly at T=2 K.
Critical requirements of a viable RF cavity superconductor material include low surface resistance, high thermal conductivity, some degree of ductility, and amenability to fabrication of large cavity shapes characterized by round (e.g., circular) plan and oval-like cross-section, commonly referred to as “pillbox” shapes. Among conventional superconductors, niobium (critical temperature TC=9.2 K) best matches these requirements; hence, state-of-the-art RF cavities are niobium-based, made either of bulk Nb or Nb-coated copper pre-form. See Vaglio, supra; E. Chiaveri, IEEE Transactions on Applied Superconductivity 13, 1199 (2003); P. Kneisel, paper at the Eleventh Workshop on RF Superconductivity, September 2003, Lubeck, Germany (available on compact disc).
Several design considerations determine the wall material, surface morphology and overall shape of the cavity, important among which are the following. Firstly, heat losses should be minimized. If heat losses are sufficiently low, operation is permitted at 100% duty cycle; superconductor material is preferred for this reason. An additional advantage of a superconductor over a metal is that a superconductor's surface resistance scales with frequency f as f2, whereas a metal's surface resistance scales with frequency f as f1/2; see Kneisel, supra. Secondly, the magnetic field intensities at the cavity walls' “hot areas” should be reduced so that they do not exceed the superconductor critical field. This consideration has led to pillbox shapes of cavities. Thirdly, “multipacting”—i.e., repeated emission of secondary electrons from imperfections at the cavity's walls—should be reduced. This consideration implies the need to eliminate cavity surface protrusions as much as possible, since it is at these protrusive locations where high local electric fields are present that lead to secondary electron emission. Consequently, the cavity's surface is meticulously treated by electrical and chemical processes, such as etching (See Vaglio, supra; Chiaveri, supra; Kneisel, supra), in order to achieve maximum smoothness. Fourthly, thermal stability should be achieved. Factors such as residual local surface imperfections and field transients can lead to localized heating that can quickly spread throughout the cavity and result in a breakdown. To efficiently diffuse these thermal instabilities, the thermal conductivity of the cavity walls should be maximized, while their surface resistance should be as low as possible. Fifthly, the “manufacturability” of cavities is of great practical importance. The material (e.g., superconductor) must be amenable to the fabrication of structures having large non-flat areas, with some degree of elasticity for mechanical adjustments.
Among conventional superconductors, niobium comes closest to the above materials requirements; see Vaglio, supra; Chiaveri, supra; Kneisel, supra. However, as previously noted herein, the low critical temperature TC of niobium mandates operation in the temperature range T=2-4 K. This low operation temperature requires a complex, bulky and expensive liquid-helium-based cooling system. Furthermore, the low thermodynamic critical field (HC=0.2 Tesla) of niobium imposes constraints on practicable cavity shapes and on maximally achievable accelerating electric field EACC.